A typical dispensing device for supplying liquid, such as hot melt adhesive, generally includes a body including a needle having a valve element that opens and closes a dispensing orifice. The needle is usually actuated in at least one direction by pressurized air to dispense discrete amounts of pressurized liquid. A spring mechanism and/or pressurized air is used to move the valve element in an opposite direction against a valve seat, which stops the flow of liquid from the dispensing orifice.
More specifically, liquid dispensing modules include a flow channel adjacent the dispensing orifice and an actuator chamber at an opposite end of the device. The actuator chamber contains a portion of the needle which is typically connected with a piston member and which may also be connected with a spring return mechanism as discussed above. Under air pressure applied on one side of the piston member sufficient to overcome the spring return mechanism, the valve element on the needle is moved in a direction away from the valve seat to discharge liquid. When the air pressure is removed, the spring return mechanism will automatically actuate the needle towards a closed position where the valve element engages the valve seat.
A dynamic seal is typically placed generally between the dispenser body and the moving needle to prevent liquid from the flow channel from leaking into the actuator chamber. Dynamic seals are understood to be seals between two surfaces that move relative to one another. These dynamic seals may press tightly against the needle and cause friction and seal wear. The higher friction may place greater demands on the requirements for pressurized air to move the needle. Even with reduced friction, the dynamic seal will wear over time and lose its ability to seal properly. This inadequate sealing will allow the liquid to enter the actuator chamber to bind the piston and will also allow pressurized air to enter into the flow channel, causing undesired dispensing discontinuities.
One alternative to a dynamic seal is a flexible bellows seal as described in U.S. Patent Application Publication No. 2006/0097015 to Riney. The flexible seal includes one end coupled to the needle and another end coupled to the dispenser body. The ends of the flexible seal can move relative to each other such that there is no frictional engagement between the seal and either of the needle and the dispenser body. It would be desirable to provide a liquid dispensing module that improves on the flexible bellows seal and further addresses the problems with using dynamic seals in a flow channel.